Lithium-ion batteries (LIBs) have attracted extensive attention in the past two decades for a wide range of applications in portable electronic devices such as cellular phones and laptop computers. Due to rapid market development of electric vehicles (EV) and grid energy storage, high-performance, low-cost LIBs are currently offering one of the most promising options for large-scale energy storage devices.
Currently, electrodes are prepared by dispersing fine powders of an active battery electrode material, a conductive agent, and a binder material in an appropriate solvent. The dispersion can be coated onto a current collector such as a copper or aluminum metal foil, and then dried at an elevated temperature to remove the solvent. Sheets of the cathode and anode are subsequently stacked or rolled with the separator separating the cathode and anode to form a battery. The separator is a physical barrier interposed between the anode and the cathode, which prevents physical contact therebetween.
The lithium-ion battery manufacturing process is sensitive to moisture. A battery with high water content leads to serious attenuation of electrochemical performance and affects stability of battery. Moisture in a battery can originate from various sources. One possible source of moisture comes from the separator. The separator may absorb moisture during manufacture, storage and transportation. It is particularly true when the separator is placed and stored in a moist environment. To address the moisture sensitive issue of electrode assembly, it is important to dry the separator before forming an electrode assembly so as to reduce the water content in the battery.
Korean Patent No. 101497348 B1 describes a method for preparing an electrode assembly. The method comprises the steps of forming a laminate by stacking a cathode, an anode, and a separator interposed between the two electrodes; heating the laminate; and pressurizing the heated laminate. The heating process melts part of the fibres of the separator in order to combine the electrodes and separator. However, this method does not dry the separator before assembling.
Korean Patent No. 101495761 B1 describes a method for preparing an electrode assembly. The method comprises the steps of preparing the negative and positive electrode plates; arranging a positive electrode plate, a negative electrode plate, and a separator to form an electrode assembly; forming a jelly roll by winding the electrode assembly; drying the jelly roll. However, this method also does not dry the separator before assembling.
Korean Patent No. 100759543 B1 describes a method for preparing an electrode assembly of a lithium-ion polymer battery. The method comprises the steps of preparing a positive electrode plate and a negative electrode plate; preparing a separator; heating the separator; and interposing the heated separator between the two electrode plates, wherein the separator is heated at an elevated temperature for 1-3 minutes. However, the heating process is used to remove the residual stress inside the separator so as to prevent shrinkage of the separator due to overheating of the battery.
The absence of a process of pre-drying separators in existing method introduces water into the electrode assemblies, which may affect the cycling stability and rate capability of LIBs. In view of the above, there is always a need to develop a method of drying separators of LIBs to low water content before assembling into an electrode assembly.